Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, synthetic diamond material can be deposited.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for synthetic diamond film growth using a CVD process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
A useful overview article by Silva et al. summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see “Microwave engineering of plasma-assisted CVD reactors for diamond deposition” J. Phys.: Condens. Matter, Vol. 21, No. 36 (2009) 364202). This article identifies that from a purely electromagnetic standpoint, there are three main design criteria: (i) the choice of the resonant mode; (ii) the choice of the coupling structure (electric or magnetic); and (iii) the choice of dielectric window (shape and location).
Having regard to point (i), Silva et al. identify that circular transverse magnetic (TM) modes, and particularly TM0mn modes, are most suitable. In this notation, the first index number (here 0) indicates that the electric field structure is axisymmetric, which will yield a circular plasma. The indices m and n represent the number of nodes in the electric field in the radial and axial directions, respectively. Silva et al indicate that a number of different modes have been used in prior art reactors including: TM011; TM012; TM013; TM020; TM022; TM023; and TM031.
Having regard to point (ii), Silva et al. identify that electric field (capacitive) coupling using an antenna is the most widely used and that magnetic (inductive) coupling is rarely used because of the limited power than can be coupled. That said, a commercially available IPLAS reactor is disclosed as using magnetic coupling to support a TM012 mode.
Having regard to point (iii), Silva et al. describe that an essential element associated with both electric and magnetic coupling schemes is a dielectric window which is generally made of quartz and delimits a reduced pressure zone inside the cavity in which reactant gases are fed to form a plasma when excited by the electromagnetic field. It is described that the use of a quartz window allows a user to select a single electric field anti-node region (of maximum electric field) such that the plasma can be ignited only in this region and the formation of parasitic plasma at other electric field maxima within the chamber can be avoided. The quartz window is conventionally in the form of a bell-jar placed over the substrate on which deposition is to occur and around an electric field anti-node located adjacent the substrate. Other dielectric window configurations are also disclosed. For example, an ASTEX reactor is described which includes a dielectric window in the form of a plate located across the reactor chamber approximately at the cavity mid-plane while a second-generation ASTEX reactor is described as having a dielectric window in the form of a quartz tube which is not directly exposed to the plasma so as to give the reactor better power handling capabilities.
In addition, the article discloses various geometries of prior art reactor chambers including: a cylindrical chamber such as the MSU reactor which is designed to support a TM012 mode, the ASTEX reactor which is designed to support a TM013 mode, or LIMHP reactor designs supporting a TM023 mode or a TM022 mode; an ellipsoidal chamber such as the AIXTRON reactor; and other non-cylindrical chambers such as the second generation ASTEX reactor which has a central cylindrical component purported to support a TM011 mode between the substrate holder and a top portion of the chamber and laterally extending side lobes supporting a TM021 mode so that the chamber as a whole supports multiple modes. The second generation ASTEX reactor has only one Ez-field maximum in the upper part of the central section of the chamber which is the case for a TM011 mode, but two Ez maxima in its lower half, as expected for a TM021 mode.
Having regard to the patent literature, U.S. Pat. No. 6,645,343 (Fraunhofer) discloses an example of a microwave plasma reactor configured for diamond film growth via a chemical vapour deposition process. The reactor described therein comprises a cylindrical plasma chamber with a substrate holder mounted on a base thereof. A cooling device is provided below the substrate holder for controlling the temperature of a substrate on the substrate holder. Furthermore, a gas inlet and a gas outlet are provided in the base of the plasma chamber for supplying and removing process gases. A microwave generator is coupled to the plasma chamber via a high-frequency coaxial line which is subdivided at its delivery end above the plasma chamber and directed at the periphery of the plasma chamber to an essentially ring-shaped microwave window in the form of a quartz ring. The invention as described in U.S. Pat. No. 6,645,343 focuses on the ring-shaped microwave window and discloses that the coupling of microwaves in the reactor chamber is distributed in circularly symmetric fashion over the entire ring surface of the microwave window. It is taught that because the coupling is distributed over a large surface, high microwave power levels can be coupled without high electric field intensities developing at the microwave window thus reducing the danger of window discharge.
As such, U.S. Pat. No. 6,645,343 addresses two of the three design criteria described by Silva et al. in their Journal of Physics article discussed previously, i.e., the choice of the coupling structure (magnetic) and the choice of dielectric window (ring-shaped dielectric widow located around the side wall of a cylindrical reactor chamber). U.S. Pat. No. 6,645,343 is silent as to what resonant mode the chamber should be design to support and what design criteria should be applied to the chamber in order to best support the desired resonance mode to achieve a uniform, stable, large area plasma across the surface of a large area substrate/holder for achieving uniform CVD diamond growth over large areas.
It light of the above discussion and the prior art mentioned therein, it will be evident that it is a well known aim in the field of CVD diamond synthesis to form a uniform, stable, large area plasma across the surface of a large area substrate/holder for achieving uniform CVD diamond growth over large areas and that many different plasma chamber designs and power coupling configurations have been proposed in the art for trying to achieve this goal. However, there is an on going need to improve upon the prior art arrangements in order to provide larger CVD growth areas, better uniformity, higher growth rates, better reproducibility, better power efficiency and/or lower production costs. It is an aim of certain embodiments of the present invention to address this on going need.